GMD Geoscientific Model Development GMD Geosci. Model Dev. 1991-9603 Copernicus Publications Göttingen, Germany 10.5194/gmd-7-2933-2014 Predicting the response of the Amazon rainforest to persistent drought conditions under current and future climates: a major challenge for global land surface models Joetzjer E. 1 Delire C. 1 Douville H. 1 Ciais P. 2 Decharme B. https://orcid.org/0000-0002-8661-1464 1 Fisher R. 3 Christoffersen B. https://orcid.org/0000-0002-4890-9999 4 Calvet J. C. https://orcid.org/0000-0001-6425-6492 1 da Costa A. C. L. 5 Ferreira L. V. 6 Meir P. 7 CNRM-GAME UMR3589, Groupe d'étude de l'atmosphère météorologique, Toulouse, France LSCE Laboratory of Climate Sciences and the Environment, Gif-sur-Yvette, France NCAR National Center for Atmospheric Research, Boulder, Colorado, USA School of GeoSciences, University of Edinburgh, Edinburgh, UK Universidade Federal de Para, Belem, Para, Brasil Museu Paraense Emilio Goeldi, Belem, Para, Brasil Australian National University, Canberra, Australia 10 12 2014 7 6 2933 2950 Copyright: © 2014 E. Joetzjer et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://gmd.copernicus.org/articles/7/2933/2014/gmd-7-2933-2014.html The full text article is available as a PDF file from https://gmd.copernicus.org/articles/7/2933/2014/gmd-7-2933-2014.pdf

While a majority of global climate models project drier and longer dry seasons over the Amazon under higher CO<sub>2</sub> levels, large uncertainties surround the response of vegetation to persistent droughts in both present-day and future climates. We propose a detailed evaluation of the ability of the ISBA<sub>CC</sub> (Interaction Soil–Biosphere–Atmosphere Carbon Cycle) land surface model to capture drought effects on both water and carbon budgets, comparing fluxes and stocks at two recent throughfall exclusion (TFE) experiments performed in the Amazon. We also explore the model sensitivity to different water stress functions (WSFs) and to an idealized increase in CO<sub>2</sub> concentration and/or temperature. In spite of a reasonable soil moisture simulation, ISBA<sub>CC</sub> struggles to correctly simulate the vegetation response to TFE whose amplitude and timing is highly sensitive to the WSF. Under higher CO<sub>2</sub> concentrations, the increased water-use efficiency (WUE) mitigates the sensitivity of ISBA<sub>CC</sub> to drought. While one of the proposed WSF formulations improves the response of most ISBA<sub>CC</sub> fluxes, except respiration, a parameterization of drought-induced tree mortality is missing for an accurate estimate of the vegetation response. Also, a better mechanistic understanding of the forest responses to drought under a warmer climate and higher CO<sub>2</sub> concentration is clearly needed.

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